Currently, assembling elongated spools into close fitting valve bodies is a manual operation. The reason for this is that clearance between spool lands and bores is .0005 inch, the lands and bores have sharp corners (no lead chamfers), and the bores may not be manufactured to a datum plane. If a bore is machined to a datum plane, then it is a simple matter to orient the bore in a fixture for a later assembly process with the bore oriented in a desired direction, since the orientation of the bore relative to the valve housing is precisely known. If a spool bore is not machined to a datum plane, it is difficult in a later assembly process to precisely align the machined bore with the insertion tooling for a valve spool so that the spool can be inserted into the bore. Even if a bore is machined to a datum plane, a method is still needed to assist the leading end of the spool to find and enter the land bores in the housing. Currently, there is no reliable method or configuration of tooling that will align spools to bores through which a spool can be dropped into a bore with clearance of .0005 inches.
One technique for facilitating the assembly of a spool into a valve bore uses an air bearing as shown in U.S. Pat. No. 5,829,134 for a Spool Valve Loading Method and Apparatus. The method and apparatus described therein uses pressurized gas at a first pressure to advance a valve spool into a valve body bore, and at the same time pressurized gas at a second lower pressure is applied to the valve body bore to create a moving cushion of air that opposes the insertion of the valve spool into the valve bore. The cushion of air from the valve body creates an air bearing that centers the spool in the bore so that the spool can be inserted the full length into the bore without hanging up on the lands of the bore.
The technique described above provides satisfactory results provided the spool bores are machined to a datum plane. If the spool bores are not machined to a datum plane, since the bores cannot be precisely aligned with the insertion tooling for the valve spools, the spools will jam if the valve body is positioned too close to the insertion tooling. As the gap between the valve body and the insertion tooling is increased, the tooling cannot maintain the alignment of the spool to the bores, with the result that the spools will jam as they are inserted into the bores, or the leading end of the spool will contact and rest on the surface around the entrance to the bore.
For the foregoing reasons, the air bearing technique, by itself, does not work for all combinations of spool lengths and spool land diameters. Additional assembly aids are needed to produce an assembly system that will successfully load spools over 95% of the time. The features described herein provide the compliance needed for the spool to be inserted into the bore in a valve body in an automated assembly operation.